The cost of aluminum production can be significantly reduced by substituting inert anodes for the carbon anodes that are used in most commercial electrolytic cells today. The inert anodes are not consumed during aluminum production so that they are dimensionally stable. The use of a dimensionally stable inert anode together with a wettable cathode also allows more efficient cell designs, lower current densities and a shorter anode-cathode distance, with resulting energy savings.
The inert anode material must satisfy several demanding conditions. For example, the material must not react with the molten salt electrolyte or dissolve in it. The material must not react with oxygen or corrode in an oxygen-containing atmosphere at the cell operating temperature. The material must be relatively inexpensive and have good mechanical strength. It must have electrical conductivity greater than about 120 ohm.sup.-1 cm.sup.-1 at the cell operating temperature. In addition, aluminum produced in a cell having inert anodes should not be significantly contaminated by constituents of the inert anode material.
Aluminum smelting cells having inert anodes must be operated at sufficiently low temperatures that the molten salt bath is not volatilized. The operating temperature must be high enough that the electrolyte does not solidify and that alumina solubility in the electrolyte is sufficient to operate the cell efficiently. In addition, low cell operating temperatures pose a risk of developing high anode resistance.
A principal objective of our invention is to provide a process for producing aluminum in an electrolytic cell having an inert anode and a molten salt bath, wherein the molten salt bath temperature is controlled to avoid attack upon the inert anode.
A related objective of our invention is to provide a process for electrolytic production of aluminum in a cell having an inert anode and a molten salt bath, wherein alumina is soluble in the molten salt bath and high anode resistance is avoided.
Additional objectives and advantages of our invention will become apparent from the following detailed description.